The development of weapons and energy plants that utilize radioactive material has given rise to a need for neutron detectors. The need to determine and know the entry of material including radioactive material requires performance of neutron surveillance at points of entry for ports, buildings, and other areas where people, cargo, or objects enter. Such neutron surveillance should be accomplished with a minimum of restriction or disruption of material flow and events.
Examples of detectors in a metastable physical system include cloud chambers and bubble detectors. In the former case, a medium consisting of a gas mixture supersaturated with a condensable vapor (e.g., water vapor) detects ionizing radiation. Although there is a higher vapor pressure of water vapor than would be possible at equilibrium, the nucleation of small water droplets in the absence of dust particles represents an energetic barrier to such equilibration, so no cloud is formed, the medium cannot reach equilibrium and, hence, is termed “metastable.” Gas ionization from radiation, however, facilitates the water vapor condensation nucleation. A single radiation particle track induces many orders of magnitude times as many molecules to condense from the gas phase and form a detectable cloud.
In a complementary example, neutron detectors have been fabricated from superheated liquid/gel emulsions. In this case, the nucleation of a gas bubble within the liquid represents a barrier, due to surface tension, preventing equilibration by boiling. In this case, a 10B(n,α)7Li disintegration event deposits enough energy into a small enough volume of this liquid that the bubble nucleation barrier is overcome, and the accumulated energy of superheating a macroscopic (detectable easily by the unaided eye) amount of liquid is released by a single disintegration event. In this case, the gain is limited only by the rheological properties of the gel and the size of the micelle. Unfortunately, neither cloud chambers nor bubble detectors can easily be used as inexpensive portal monitors for neutrons emitted by special nuclear material.
The effectiveness of neutron detectors is, in part, determined by their ability to discriminate neutron radiation from other types of radiation, such as gamma or beta radiation. Because the latter interact strongly only with heavy or “high-Z” elements, and create signals that may obscure neutron signals, there is much interest in development of “low-Z” neutron detectors comprised mostly of less massive elements, i.e., organic polymers or organic molecular crystal detectors, which respond robustly only after a neutron absorption event that creates energetic massive particles. On the other hand, detectors that induce phase transitions are expected to be more sensitive to deposited energy density than those that measure charge created by ionizing radiation, and so it is not clear that a high-Z metastable material will not also be highly discriminating. What is clear in both of the above examples (cloud chamber and bubble detector), is that the nucleation of such equilibration pathways, whether in a low-Z or high-Z medium, becomes more likely as the energy deposition of the energetic particle becomes more dense and, therefore, these techniques are most effective when more massive (alpha and heavier) particles are created upon neutron capture.
A separate class of neutron detector senses light emitted from a scintillator. Scintillators have been developed for both fast and slow neutron detection. Fast neutrons have a higher cross section for collisions with protons that subsequently transfer energy to the scintillator, which promptly emits light. Moderated (slow) neutrons, on the other hand, have a higher cross section for capture by nuclei such as 3He, 6Li, and 10B and for these nuclei, neutron capture disintegrations result that produce heavier energetic particles, such as a 1.7 MeV alpha particle, in the case of the 10B capture, that transfer energy to the scintillator. The light from a scintillator is detected by a photomultiplier tube to generate an electrical pulse proportional to the number of photons created by the energetic particle(s). Scintillator systems' potential for large area neutron detectors, such as required for portal monitoring, is limited in that the cost of the many photomultiplier tubes required for an effective detector becomes prohibitive.
The most common type of thermal neutron detector is a gas filled counter, typically using pressurized gaseous helium-3 filled tubes. These 3He detectors are delicate and occasionally indicate false positive signals when abruptly moved or impacted. Although effective, such types of neutron detectors are generally not suitable for operations requiring devices capable of functioning for long periods with low power consumption. 3He is also extremely expensive. Therefore, proliferation of such devices to anything other than primary portals would be prohibitively expensive.
Yet another type of neutron detector is a solid state electronic device that can sense alpha particles emitted from a neutron converter material in which an (n, α) reaction has taken place. The converter material is required to convert incident neutrons into emitted charged particles, which are more readily sensed. When the emitted charged particle transits a semiconductor device, the charged particle liberates charges in its wake or path that may be collected and used to sense the event stimulated by the initial neutron reaction. Such devices serve as neutron detectors. In demonstrations of such a device, free-standing converter foils are placed near a silicon detector, such as a PIN diode. Typically, films of converter material are place in contact with or deposited directly upon semiconductor detectors. Lithium metal has been used for this purpose, although the chemical reactivity of the lithium metal leads to shorter detector life. A longer life for such devices has been achieved using compounds of lithium, such as lithium fluoride, which is a hard crystalline-type material. Boron metal has also been used as a coating on semiconductor devices.
Semiconductor memory cells, which are susceptible to single event upsets or single event errors when an incoming particle induces an error in the memory cell have been used to detect neutrons. Attempts have been made to use commercial memory circuits with a neutron converter so that single event upsets detect neutrons. In manufacturing semiconductor devices, boron has been used as a dopant and used in glass as a passivation layer used either to cover the circuit structures or to encapsulate a completed semiconductor device. Boron-10 in the dopant or borophosphosilicate glass (BPSG) passivation layer is responsible for sensitizing a circuit to neutron radiation.
U.S. Pat. Nos. 7,271,389; 6,867,444; 6,841,841; and 6,075,261 disclose attempts to utilize a conventional semiconductor memory device as a neutron detector using a neuron-reactant material (converter) coated over a conventional Flash memory device. Alpha particles emitted by the boron typically must pass through structural layers that define the circuit before they reach an active semiconductor, which results in insensitive detectors because the boron conversion material is not located close to the active semiconductor material layer. This causes alpha particles generated by the boron conversion material to dissipate their energy in the intervening structural material so that a sufficient charge in the active semiconductor material layer to cause single event “upsets” cannot be generated. Hence, it is important for the detecting element and the boron-containing element to be one and the same, and this precludes silicon-based devices, because the boron would render the silicon conductive and short the circuit.
Therefore, in view of problems and limitations associated with current neutron detectors, it would be useful to have a neutron detector that does not require the use of high pressure tubes of expensive 3He or photomultiplier tubes, that operates at low power consumption, that can sense with high gain individual neutron events, and that is not sensitive to other types of radiation.